Re: [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files

[Barret Rhoden]

Hi -

On 12/12/19 1:49 PM, Liran Alon wrote:
> > On 12 Dec 2019, at 20:47, Liran Alon <liran.alon@xxxxxxxxxx> wrote:
> >
> >
> >
> > > On 12 Dec 2019, at 20:22, Barret Rhoden <brho@xxxxxxxxxx> wrote:
> > >
> > > This change allows KVM to map DAX-backed files made of huge pages with
> > > huge mappings in the EPT/TDP.
> >
> > This change isnât only relevant for TDP. It also affects when KVM use shadow-paging.
> > See how FNAME(page_fault)() calls transparent_hugepage_adjust().

Cool, I'll drop references to the EPT/TDP from the commit message.

> > > DAX pages are not PageTransCompound.  The existing check is trying to
> > > determine if the mapping for the pfn is a huge mapping or not.
> >
> > I would rephrase âThe existing check is trying to determine if the pfn
> > is mapped as part of a transparent huge-pageâ.

Can do.

> > > For
> > > non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound.
> >
> > This is not related to hugetlbfs but rather THP.

I thought that PageTransCompound also returned true for hugetlbfs (based 
off of comments in page-flags.h).  Though I do see the comment about the 
'level == PT_PAGE_TABLE_LEVEL' check excluding hugetlbfs pages.

Anyway, I'll remove the "e.g. hugetlbfs" from the commit message.

> > > For DAX, we can check the page table itself.
> > >
> > > Note that KVM already faulted in the page (or huge page) in the host's
> > > page table, and we hold the KVM mmu spinlock.  We grabbed that lock in
> > > kvm_mmu_notifier_invalidate_range_end, before checking the mmu seq.
> > >
> > > Signed-off-by: Barret Rhoden <brho@xxxxxxxxxx>
> >
> > I donât think the right place to change for this functionality is transparent_hugepage_adjust()
> > which is meant to handle PFNs that are mapped as part of a transparent huge-page.
> >
> > For example, this would prevent mapping DAX-backed file page as 1GB.
> > As transparent_hugepage_adjust() only handles the case (level == PT_PAGE_TABLE_LEVEL).
> >
> > As you are parsing the page-tables to discover the page-size the PFN is mapped in,
> > I think you should instead modify kvm_host_page_size() to parse page-tables instead
> > of rely on vma_kernel_pagesize() (Which relies on vma->vm_ops->pagesize()) in case
> > of is_zone_device_page().
> > The main complication though of doing this is that at this point you donât yet have the PFN
> > that is retrieved by try_async_pf(). So maybe you should consider modifying the order of calls
> > in tdp_page_fault() & FNAME(page_fault)().
> >
> > -Liran
>
> Or alternatively when thinking about it more, maybe just rename transparent_hugepage_adjust()
> to not be specific to THP and better handle the case of parsing page-tables changing mapping-level to 1GB.
> That is probably easier and more elegant.

I can rename it to hugepage_adjust(), since it's not just THP anymore.

I was a little hesitant to change the this to handle 1 GB pages with 
this patchset at first.  I didn't want to break the non-DAX case stuff 
by doing so.

Specifically, can a THP page be 1 GB, and if so, how can you tell?  If 
you can't tell easily, I could walk the page table for all cases, 
instead of just zone_device().

I'd also have to drop the "level == PT_PAGE_TABLE_LEVEL" check, I think, 
which would open this up to hugetlbfs pages (based on the comments).  Is 
there any reason why that would be a bad idea?

Thanks,

Barret